Effect of carbon coating on electrochemical performance of LiFePO4 cathode material for Li-ion battery

Pristine LiFePO4 (LFP) and carbon-coated LiFePO4 (LFP/C) are synthesized by sol-gel process using citric acid as a carbon precursor. LFP/C is prepared with three different stoichiometric ratios of metal ions and citric acid, namely 1:0.5, 1:1, and 1:2. Prepared LFP and LFP/C powder samples are characterized by X-ray diffractometer, field emission scanning electron microscope, transmission electron microscope, and Raman spectrophotometer. Electrochemical performances of pristine and carbon-coated LFP are investigated by charge-discharge and cyclic voltammetry technique. The results show that LFP/C (1:1) with an optimum thickness of 4.2 nm and higher graphitic carbon coating has the highest discharge capacity of 148.2 mA h g(-1) at 0.1 C rate and 113.1 mA h g(-1) at a high rate of 5 C among all four samples prepared. The sample LFP/C (1:1) shows 96% capacity retention after 300 cycles at 1 C rate. The decrease in discharge capacity (141.4and 105.9 mA h g(-1) at 0.1 and 5 C, respectively) is observed for the sample LFP/C (1:2). Whereas, pristine LFP shows the lowest discharge capacity of 111.1 mA h g(-1) at 0.1 C and capacity was decreased very fast and work only up to 147 cycles. Moreover, cyclic voltammetry has also revealed the lowest polarization of 0.19 V for LFP/C (1:1) and the highest 0.4 V for pristine LFP.